1. Field of the Invention
The present invention relates to apparatus and method for detecting defects in a storage medium and, more particularly, for detecting defects in a high density storage medium such as a magnetic disk.
2. Description of the Related Art
Magnetic storage media, such as hard disks or floppy disks, are comprised of a substrate upon which is coated a thin layer of magnetic material. Small defects or flaws can exist in the thin film layer of magnetic material on a disk or the disk substrate, e.g., pits, plating pin holes, and residual polishing scratches which cannot be removed by a texturing process. Such defects can result in writing and reading of erroneous data bits. Such erroneous bits are created when data is written into a defective area of the disk and subsequently read out from the disk. A data bit error for a particular bit is caused either by the magnetization of a bit being missing or by demagnetization being added at the storage location.
In order to test and identify defects in the layer of magnetic material or disk substrate, a typical prior art technique used by manufacturers of magnetic media is to perform a surface analysis of the layer of magnetic material and produce an error map for the surface of the disk. The error map is then written on the disk for future reference to avoid the defective areas of the disk during subsequent recording and playback of data. Alternatively, if the disk contains an unacceptable number of defects, it may be wholly rejected.
Various techniques are known for performing the surface analysis. One technique for performing the surface analysis involves writing a test signal such as a high frequency, alternating data pattern onto the disk. This pattern is then read from the disk as a high frequency output test signal which has a sinusoidal waveform consisting of sinusoidal data pulses corresponding to the recorded data bits of the test signal. The sinusoidal data pulses are monitored for deviations from the expected waveform of the pulses to indicate the occurrence of a defect on the disk. For example, the peaks of the output data pulses may be compared to a threshold amplitude. If a peak amplitude of an output pulse is less than the threshold, the location on the disk corresponding to the pulse is determined to contain a defect. In accordance with another technique, the phase of the output test signal is monitored. An output data pulse that occurs with a shifted phase, i.e., occurs outside an expected time window of occurrence, may be determined to correspond to a defect in the disk.
A summary of such conventional defect testing techniques is disclosed in U.S. Pat. No. 4,929,894.
Separately, developments in magnetic recording media materials and media manufacturing techniques as well as read/write head and disk drive design, have resulted in continuously increasing recording capacities. These increasing capacities correspond to an increase in the recording density of magnetic media as expressed in recordable bits per unit area, e.g., gigabits per square inch (Gbits/in.2). For example, recent recording density requirements have reached 8 Gbits/in.2. With this increase in recording density, the size of a defect in the layer of magnetic material that can cause an error has correspondingly decreased.
As recording density increases, the time required for a manufacturer of magnetic media to test the integrity of the media also increases. The density of data per unit area, i.e., areal density, for which the media must be capable can be expressed as: Areal Density=Bit Densityxc3x97Track Density. Thus, manufacturing throughput may be reduced as a result of increased recording density. One solution that has been practiced to obviate this difficulty is to test less than 100% of the magnetic media surface for defects. The result of this solution is a magnetic disk that may contain defects that cause data errors during operation. This, in turn, may cause customer dissatisfaction and decreased sales.
Another solution is to use a wider read/write head to test the integrity of the magnetic medium than would be required to write and read data with the intended data recording density. This solution results in a reduced sensitivity to defects during performance of the surface analysis with the result that smaller defects may not be detected even though such smaller defects may nevertheless be sufficiently large to cause read/write errors for the intended high recording density application. Again, the result is a magnetic disk that may contain defects that cause data errors during operation.
Additionally, malfunctioning of apparatus for testing the integrity of the magnetic medium may lead to failure to detect defects or falsely indicating defects. The former case may result in selling a magnetic disk that causes data errors during operation and consequent customer dissatisfaction. The latter case may result in a decision to reject an otherwise acceptable disk.
Accordingly, the present invention is directed to apparatus and method for testing storage media, including magnetic storage media, that substantially obviates one or more of the problems due to limitations and disadvantages of the prior art.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided apparatus for reading a plurality of information tracks in a magnetic storage medium, comprising: a read head including a plurality of read elements each producing a read signal in response to data stored in the tracks of the storage medium; and an element monitoring circuit to monitor the plurality of elements and provide an indication when one of the elements malfunctions.
Also in accordance with the present invention there is provided apparatus for reading a plurality of information tracks in a magnetic storage medium, comprising: a read head including a plurality of read elements each producing a read signal in response to data stored in the storage medium; means for storing ones of the read signals output by a selected one of the read elements; and means for comparing at least one of the stored read signals with at least one subsequent read signal output by the selected read element, for determining whether the selected read element malfunctions, and for providing an indication when the read element malfunctions.
Further in accordance with the present invention there is provided a method for reading a plurality of information tracks in a magnetic storage medium, comprising: producing read signals from a plurality of read elements in response to data stored in the track of the storage medium; monitoring the plurality of read elements to determine when any one of the read elements malfunctions; and providing an indication when any one of the read elements is determined to be malfunctioning.
Additionally in accordance with the present invention there is provided a method for reading a plurality of information tracks in a magnetic storage medium, comprising: producing read signals from a plurality of read elements in response to data stored in the storage medium; storing ones of the read signals output by a selected one of the read elements; and comparing at least one of the stored read signals with at least one subsequent read signal output by the selected read element; determining whether the selected read element is malfunctioning based on a result of the comparing; and providing an indication when the selected read element is malfunctioning.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.